1. Field of the Invention
The present invention relates to a method and device of measuring a physical quantity.
2. Related Art Statement
In an application of mounting a vibratory gyroscope on a vehicle, the gyroscope is used in an extremely wide temperature range. For example, it is required that the gyroscope properly functions in a wide temperature range of −40° C. to +85° C. Even when the resonance frequency of a pair of bending vibration arms is adjusted at a constant quantity at room temperature, the change and deviation of the resonance frequency may be large as the temperature is considerably changed to a high or low temperature. The change or deviation of resonance frequency thus results in a so-called zero point temperature drift.
The assignee filed a Japanese Patent publication 2000-107725A, and disclosed that tapered parts are provided at both roots of a bending vibration arm on both sides, respectively, to reduce the zero point temperature drift.
The inventors have studied such technique and found the following problems, depending on the material of the vibrator. That is, as described in Japanese patent publication 2000-107725A, tapered parts are provided in the roots of the side faces of a bending vibration arm so that the tapered parts have substantially the same planar shape. It is considered that the vibration mode of the bending vibration arm may be made more symmetrical to reduce the zero point temperature drift. When the zero point temperature drift is measured for each of the vibrators actually fabricated, however, the drift may deviate among the vibrators. The deviation of zero point temperature drift may be increased so that the yield of off-specification products is increased.
The reasons are as follows. Even when the zero point temperature drift of the vibrator cannot be made zero, it is possible to substantially cancel the drift by providing a correction circuit of the drift in a detection circuit of a vibratory gyroscope when the drift is made substantially constant. Even if the deviation of the drift of the vibrator actually fabricated is larger, the drift may be cancelled by means of the correction circuit in one vibrator. In this case, however, the drift in another vibrator may be considerably different from that in the one vibrator in which the drift is cancelled with the circuit. It is not possible to cancel the drift in another vibrator so that the performance of a vibratory gyroscope having this vibrator may be made out of specification.
The assignee filed a Japanese Patent application 2002-68862A and found the following. In an etching step, a photoresist is applied on both of upper and lower faces of a wafer. Photo masks are mounted on the photoresists, and the photo masks on the upper and lower faces are aligned with each other. The photoresist are exposed to light to harden it, and the photo masks are removed to complete the patterning of the photo resists. The wafer is then etched so that an outline corresponding with the pattern of the photoresist is formed on the wafer. When the photo masks are mounted on both of the upper and lower faces of the wafer and aligned with each other, alignment error may occur between the photo masks. In this case, when a bending vibration is excited in a bending mode along a plane of the vibrator, an unnecessary vibration component in the direction of Z-axis perpendicular to the plane is induced at the same time. Such kind of the unnecessary vibration component in Z-axis is induced in a bending vibration arm for detection at the same time to result in a noise in the detecting signal.
According to a Japanese Patent application 2002-68862A, it is disclosed that the cross sectional shape of each bending vibration arm is made a specific elongate shape to prevent the above problems. According to the method, however, the design of the thickness of the wafer is considerably limited.